Thermionic tube



June 10, 194']. WQLFF 2,244,752 THERMIONIC TUBE Filed Jan. :51, 1939 H K k 5 E r 5 5/ 1/25 Z5 M v Zhmentor i fi'ulgglifrg EH81; P MP attorney Patented June 10, 1941 THERMIONIC TUBE Irving Wolff, Mcrchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1939, Serial No. 253,731

2 Claims.

This invention relates to thermionic tubes, and more particularly to tubes in which the cathode electrode is subject to severe bombardment which results in excessive heating, and has for its principal object the provision of a method of and means for improving the stability of performance of such tubes. Another object is to provide an improved control of electron emission. A further object is to prevent damage to the filament or cathode electrode due to excessive heating resulting from bombardment. A still further object is to provide an improved thermionic tube.

The problems relating to the control of electron emission in certain types of thermionic tubes are discussed in my copending application Serial No. 113,184, filed November 28, 1936, entitled Current or voltage regulators.

It is well known that under certain conditions Barkenhausen-Kurz, magnetron, or other electronic oscillators, for example, may reach a condition in which the temperature of the cathode suddenly begins to increase. This heating frequently reaches such proportions that the cathode is maintained at incandescence when the cathode heating current has been stopped, and may even be sufficient to damage the cathode. While the cause of this-phenomena is not clearly understood, it is generally agreed that it is due to the bombardment of the cathode by electrons or ions or a combination of'both.

Inasmuch as the frequency of oscillation of a magnetron, as well as its amplitude, is a function of electron emission, and since electron emission, in turn, depends on the temperature of the cathode, it is apparent that a regenerative action occurs in which the bombardment increases the temperature of the cathode which, in turn, increases electron emission, and the increased electron emission reacts on the cathode by increasing the bombardment. Any change in the operating conditions of such a tube which has a tendency to decrease or increase this bombardment is amplified by this action, with the result that oscillation or amplification becomes unstable.

In accordance with the above identified copending application, the problem is considered from the point of view of controlling the cathode energizing current as a function of cathode electron emission. As a result, the system therein shown decreases the energizing current when cathode emission increases for any reason. The limit of this control is reached when the cathode energizing current has been reduced to zero. I have found, however, that such a control is not always sufficient. Having once started the process of bombardment, the cathode energizing current may be completely removed and the cathode temperature will be maintained solely by the heating effect of the bombardment. When the cathode emission is maintained in this manner, energy is actively being taken from the anode supply to heat the cathode. In fact, this may reach such proportions that the filament is completely burned out.

It is the purpose of this invention to prevent such an occurrence. This is accomplished by providing a cooling system which will increase the thermal dissipation of the cathode to a value which is equal to or greater than the thermal accumulation due to the bombardment. This system may be applied to a thermionic tube alone or in combination with the electron emission control system referred to above.

This invention will be better understood from the following description when considered in connection with the accompanying drawing, in which Figure 1 represents an embodiment of this invention; Figure 2 is a cross-sectional view of a cooled cathode structure in accordance with a modification of this invention; Figure 3 illustrates an arrangement using a modified cathode; and Figure 4 is a cross-sectional view of a modification of the cathode structure shown in Fig. 3. Similar reference numerals refer to similar parts throughout the drawing.

Referring to Fig. 1, a thermionic tube 3 is shown which, by way of example, may be a magnetron. The tube consists of a pair of semicylindrical anode segments 5 and 1 which are suitably connected to an external tank circuit 9. The cathode consists of a filament H which has a spiral shape and is axially positioned between the two anode segments. The filament H is energized by a battery [3, or the like, which is connected in series with a current regulator [5, of the type described in the above identified copending application, for example. The oscillatory circuit is completed by a connection from one terminal of the cathode II to the midpoint of the resonant circuit 9 through a source of anode potential I! and a radio frequency choke IS. A magnetic field is produced by passing a current through coil 2| by means of a battery 23.

The structure so far described is substantially identical to that of any well known magnetron oscillator. In accordance with one embodiment of this invention, the thermal dissipation of the cathode is increased by passing a cooling fluid through a fluid conducting tube 25 which is within and adjacent to the spiraled cathode I I. The tube is suitably electrically insulated so that it will not short circuit the cathode. This tube may be a metal tube which is provided with openings to an external system through which a cooling fluid may be passed. If desired, a pump 21 may be provided for aiding the circulation of the cooling fluid.

In view of the high temperatures involved, provision must be made to use a fluid which is not afiected by high temperatures. For example, a liquid having a high boiling point is desirable. A suitable liquid of this type is glycerine. It is desirable to prevent the temperature of the liquid from reaching a high value either by providing partial thermal insulation between the cathode and the liquid or by circulating the liquid through the cathode at a high rate.

A normally gaseous fluid may also be used to cool the cathode. Since it is desirable to prevent chemical reaction between the cooling fluid and the tube 25, it is necessary to exercise a proper choice of material for the tube or to utilize an inert gas selected from the group argon, crypton, Xenon or nitrogen.

The modification mentioned above in which a partial insulator is placed between the cooling fluid and the cathode is illustrated in Fig. 2. The tube 25 is shown as before. It is surrounded by an insulating cylinder 29 which is a relatively poor heat conductor and around which is wound the helical cathode H.

Fig. 3 illustrates a modified cathode arrangement in which the heating element is itself a part of the fluid conducting pipe 25, thus eliminating the spiral filament H. As shown in a very much enlarged view in Fig. 3, the active cathode 3i is hollow and the cooling fluid from the cooling system passes directly through it. The cathode 3! is energized by inducing 2. current in the cooling coil 33 from the primary 35. An iron core 3'! is used to increase the transfer efiiciency.

Fig. 4 is a cross-sectional view of an enlarged cathode segment of the type shown in Fig. 3 which is provided with an internal thermal insulating jacket 39. This jacket may consist of any poor heat conductor, and may, for example, be a confined gas.

The stability of operation of an oscillator of this type is increased in two ways. First, since a certain thermal lag has been introduced in the cathode, its temperature is prevented from responding immediately to changes in the bombardment. Second, since the thermal dissipation has been increased greatly beyond that normally obtained by a filamentary cathode, the maximum temperature which may be reached is thereby limited. This prevents the temperature of, the cathode from increasing greatly above its normal value and thus the stability is increased.

While I have described this invention in relation to its application to a magnetron oscillator, it is apparent that the invention is not limited thereto, but is applicable to magnetron amplifiers and detectors as well as other thermionic tubes in which a similar condition exists.

While I have shown my invention in combination with a constant emission cathode current regulator, it is not necessarily used in-this combination. Likewise, the thermal dissipation may be increased by providing a thermally conductive central core for the cathode which has suflicient radiation area to provide the desired dissipation.

I claim as my invention:

1. A. thermionic discharge device which includes a cylindrical anode electrode, a hollow cathode electrode substantially concentrically positioned within said anode, means for heating said cathode to produce a flow of electrons therefrom, means for establishing a magnetic field parallel to the common axis oi said electrodes whereby electrons are caused to rotate about and bombard said cathode, said bombardment tending to produce excessive heating thereof, and means for passing a cooling fluid through said cathode to stabilize the temperature of said cathode.

2. A thermionic discharge device which includes a cylindrical anode electrode, a tubular member extending concentrically through said anode, said tubular member having a portion within said anode coated with an electron emissive material, means for passing a current through said tubular member to heat said material and cause electrons to be emitted therefrom, and means for passing a cooling fluid through said tubular member to prevent excessive heating due to bombardment by said electrons.

IRVING WOLFE. 

